Many golf balls are commercially selling, but they are typically classified into solid golf balls such as two-piece golf ball, three-piece golf ball and the like, and thread wound golf balls. The solid golf ball consists of a solid core of molded rubber material and a cover of thermoplastic resin (e.g. ionomer resin) covering on the solid core. The thread wound golf ball consists of a solid or liquid center, a thread wound layer formed on the center and a cover of ionomer resin or balata etc. having a thickness of 1 to 2 mm covering on the thread wound layer.
The thread wound golf ball, when compared with the solid golf ball, has better shot feel at the time of hitting and better controllability at approach shot. The thread wound golf ball is generally approved of or employed by high level golfers, especially professional golfers, who regard the characteristics of the thread wound golf balls as most important. On the other hand, the thread wound golf ball is inferior in flight distance to the solid golf ball. Therefore, it is required to provide the thread wound golf ball having sufficient flight distance, while keeping the advantage of having good shot feel and excellent controllability.
In the thread wound golf balls, there are two types, such as one comprising a solid center formed from integrally molded rubber material and the other comprising a liquid center composed of a hollow rubber sphere and liquid encapsulated in the sphere. The thread wound golf ball comprising the solid center has the advantage of having no deterioration of performance depending on temperature change, although the thread wound golf ball comprising the liquid center has poor rebound characteristics at low temperature.
Japanese Patent Kokai Publication No. 253236/1997 suggests that a thread wound golf ball comprises a solid center of two-layer structure composed of an inner center and an outer center. The inner center has a Shore D hardness of 10 to 35 and a diameter of 18 to 28 mm, the outer center has a Shore D hardness of 36 to 63, the hardness of the outer center is larger than that of the inner center by not less than 5, and the solid center has a diameter of 29 to 39.5 mm. The thread wound golf ball has short flight distance, because the diameter of the inner center is small and spin amount is high. The thread wound golf ball has poor shot feel, because a thickness of the outer center having high hardness is large.